In traditional sake brewing, a fermented product is filtered and then subjected to a heat treatment. This heat treatment is necessary to prevent putrefaction by hiochibacteria. Recently, in order to serve the fresh taste of sake without heat treatment to consumers, it has been proposed to subject sake to freezing instead of the heat treatment for preventing "hiochi" putrefaction, and to market it in a frozen state (see Japanese Patent Kokoku Nos. 48-18832 and 49-4389).
In the case of marketing frozen sake, the frozen state in a container when it is consumed is of importance. That is, it is necessary that frozen sake is quickly and uniformly thawed, and consumed in a uniform state of alcohol, water and other ingredients without any defect of taste.
When sake is frozen, in general, fine pieces of ice are formed at about -15.degree. C. In order to meet requirements of frozen storage, sake should be cooled to about -20.degree. C. and, at this temperature, much water is frozen and sake becomes a sherbet like state. At about -40.degree. C., more ice is formed and texture becomes harder, while alcohol is not frozen. Frozen sake is produced by filling the filtrate of a fermented product without heat treatment in containers and then subjecting them to freezing. Accordingly, when a temperature reaches to -40.degree. C. or lower, breakage of containers due to ice is increased, although it depends upon the material and shape of containers.
Then, freezing methods which can be applied to the production of frozen sake having the above characteristics should be studied.
Conventional freezing methods can be divided into quick freezing and slow freezing. However, the conventional methods are not suitable for freezing sake as explained below.
(1) Quick freezing
In general, conventional quick freezing is carried out by quickly freezing a product with a cooling medium e.g., liquid nitrogen of -195.degree. C. at a temperature lower than a pre-determined desired product temperature, e.g., -20.degree. C. However, sake contains much water and, upon freezing, it remarkably increases in volume due to formation of ice and, therefore, when a freezing temperature becomes lower, more increased head space volume in containers is required. In this regard, it is very difficult to provide increased head space volume of containers only to a product to be frozen from a practical viewpoint in this field. Then, when supercooling takes place, the content spouts out and further breakage of containers occurs. Therefore, it is difficult to use a cooling medium having a low temperature.
Further, when freezing is completed so quickly such as within 30 minutes, needle ice crystals are quickly formed toward the radial direction from the peripheral to the center of a container and a liquid portion having a high alcohol content remains in the center. Thus, separation of water and alcohol causes and appearance becomes whitish, which results in lack of refreshing feeling and less commercial value.
Furthermore, since sake contains 80 to 85% of water and has a large specific heat as that of water, freezing with a cooling medium such as liquid nitrogen is expensive.
Accordingly, conventional quick freezing is not suitable for sake freezing.
(2) Slow freezing
When sake is slowly frozen, water changes into small ice pieces and they gradually float up toward the upper portion of a container after a product temperature reaches to 0.degree. C. or lower to form a mass of ice, and a liquid portion having a higher alcohol content remains at the lower portion. Thus, the product separates into two portions.
Such a product has bad appearance and less commercial value. Further, when it is consumed, the ice portion thaws slowly. Therefore, in many cases, only the remaining liquid portion wherein water content is lowered and ingredients are out of harmony is firstly consumed, and subsequently a portion having higher water content formed by thawing of ice is consumed, which results in bad taste and less commercial value.
In general, in the case of freezing sake in a refrigerator, cooling and air circulation are stopped when a temperature about a temperature sensor in the refrigerator reaches a desired predetermined termperature such as -20.degree. C. Therefore, air contacting with a product does not move and air is rather warmed by the product which is in the course of cooling, which results in lowering of cooling effect. In addition, a cooling device does not work again until almost all air in the refrigerator is warmed higher than the desired temperature by convection to actuate the sensor. As the result, cooling is carried out intermittently and is not suitable for freezing sake.